Conventional apparatuses for receiving data applied to portable telephones include, for example, "Data Receiver" disclosed in Japanese Patent Laid-Open No. 166332/1988. According to the conventional apparatus for receiving data, the path memory length of a Viterbi decoder is varied depending upon the channel state. Specifically, the path memory length of the Viterbi decoder is varied depending upon the quality of the channel, and, when the quality of the channel is good, the delay within the Viterbi decoder is reduced to improve the transmission efficiency.
In the conventional apparatus for receiving data, however, the number of states in the Viterbi decoder is not taken into consideration, and, even when the transmission quality of the channel is good, the number of states is used which matches the average channel state. In other words, even when the transmission quality of the channel is good, the computation quantity of the Viterbi decoder cannot be reduced, resulting in increased power consumption, and, at the same time, the computation quantity cannot be minimized, unfavorably making it impossible to conduct high speed processing.